The Study On High-power All-fiber Near-infrared Supercontinuum Generation

Supercontinuum (SC) has wide and important applications in spectroscopy, fibercommunication, optical coherence tomography and optical metrology owing to itsbroadband width and good coherence. In recent years, high power SC has becoming ahotspot across the world with the development of diode pumping and fiber fabricationtechnology. Improving the average output power, broadening the band width andflatting the output spectrum will become the direction of SC’s future development. Thispaper carried out a comprehensive theoretical and experimental research on thegeneration of high-power all-fiber near-infrared SC which contains the followingseveral aspects:1. The idea to realize high-power SC with large mode area, double clad gain fiberis proposed in a creative way which breaks through the traditional method of SCgeneration using high nonlinear fiber pumped by a fiber laser. The analysis and designof high-power all-fiber near-infrared SC is carried out, and the limiting factors thatprevent the scaling of the average power of SC from PCF are discussed in terms of thecoupling between the pump laser and PCF, the diameter of the PCF and the diode laserpump to SC conversion efficiency. The feasibility of using fiber amplifier to realizehigh-power SC is verified in terms of the principles and advantages of the formation ofSC in a fiber amplifier, and a detailed plan to achieve hundreds-watt level all-fibernear-infrared SC is given.2. Some simulation research about the formation and propagation of near-infraredSC in a fiber amplifier is carried out, and the power limit of the near-infrared SC from afiber amplifier is analyzed and calculated. At first, the complex Ginzburg-Landauequation is chosen as the theoretical model. Then, the influences of the shape of the gainspectrum, small signal gain, the length of the gain fiber, and the pulse width as well asthe initial chirp of the input laser exerted on the nonlinear effects in the fiber amplifierare analyzed in detail. At last, the maximum output power of the near-infrared SCgenerated from a fiber amplifier is calculated after analyzing the effects of heattreatment, nonlinear effects, damage of the fiber end face, power limit of thesemiconductor diode laser and fiber combiner, and the damage of the gain fiber.3. The experimental research about the suppression of the nonlinear effects in afiber amplifier under high pump power is carried out, and a high power picosecond fiberlaser with157W average output power is achieved. To begin with, a self-made all-fiberpackage for semiconductor saturable absorber mirror (SESAM) is developed in order toovercome the flaws of commercially available package. Then, a picosecond and ananosecond fiber laser seeds are constructed in linear and ring cavity respectively, andthe control of the initial chirp of the nanosecond pulse is realized by narrowing the spectral width. Finally, the suppression of nonlinear effects in a fiber amplifier can beachieved by shortening the length of the gain fiber and improving the repetition rate ofthe input pulse, and an all-fiber picosecond fiber laser with157W average output powerand0.6nm linewidth is developed by using these two methods.4. Hundreds-watt level all-fiber near-infrared SC with high efficiency isaccomplished. After solving the problems of amplified spontaneous emission (ASE) anddamage of the gain fiber, SC with average output powers as high as70W and177.6Ware realized in succession based on a nanosecond fiber laser seed with narrow line widthand fiber amplifiers in master oscillator power amplifier (MOPA) configuration. Themagazine <Laser Focus World China> has reported the results twice in succession andregards the power as the highest across the world. The177.6W SC stretches from1064nm to2000nm with10dB spectral width of740nm. The average spectral powerdensity across the whole continuum is more than150mW/nm and the976nm diode laserpump to SC conversion efficiency is56%, which is the highest according to thepublished paper by far and is listed in―2012China Optics Outstanding Achievements‖.